1. To rot: To decompose and become soft, crumbly, or liquefied, or make something do this.

2. To decline in quality gradually and steadily, or make something do this.

3. To undergo spontaneous disintegration.

4. To decrease gradually in magnitude.

5. To decrease gradually in altitude.

1. The corpse is in an advanced decay.
2. Because they kept mixing their coke with sugar, the quality decayed.
3. The fosfor decayed.
4. Refers to a physical quantity or effect.
5. When in space, a satellite starts descending, orbit decays.